1. Field of the Invention
The present invention relates to a method for manufacturing a high strength rail with good toughness, and more particularly to a heat treatment carried out after hot rolling.
2. Description of the Prior Art
As a method of manufacturing very effectively a high strength steel rail having a fine pearlitic structure, there has been developed a method wherein a high strength rail can be obtained by charging a rail into a cooling apparatus after having subjected it to hot rolling and, thereafter, subjecting a rail head to forced cooling. In this method, cooling means applied after the hot rolling is only different from a conventional natural cooling and a toughness of the rail almost equal to that of an ordinary rail can be obtained.
In recent years, not only an outstanding wear resistance, but also a high toughness has been required for a high strength rail from a viewpoint of safety in a railway service. As a method for manufacturing the high strength rail having a good toughness with comparatively high productivity, there is pointed out a method wherein an attempt is made to make austenite grains of the rail finer at the time of roll finishing by controlling a rolling temperature and a draft during hot rolling, that is, by means of controlled rolling and by subjecting the rail to a forced cooling immediately after the finish of the rolling. In this method, however, there are problems mentioned below:
(a) In case of carrying out the controlled rolling, the rolling temperature is controlled. Therefore, a stand-by natural cooling is required during the rolling. In consequence, production efficiency in the controlled rolling decreases remarkably compared with that in ordinary rolling.
(b) In the controlled rolling, the rolling is carried out at a temperature of austenite low temperature zone. Therefore, accuracy of a rolled shape of the rail is hard to be obtained due to complicated shapes of the rail.
(c) In the rolling carried out at a temperature of the austenite low temperature zone, there is a possibility of breakdown of a roll due to an increase of a rolling load.